The invention relates to improvements in fluid flow machines in general, and more particularly to improvements in canned motor pumps wherein a so-called can (normally a high-quality tube made of steel or the like) is installed between the driving and driven units of the motor which rotates the pumping element or elements.
Canned motor pumps are known in a variety of forms and sizes and are used for a variety of different purposes. For example, such pumps can be used to circulate liquids at elevated temperatures, to convey chemical substances and/or under many other circumstances when the conveyed fluid media are toxic, radioactively contaminated, hot, corrosive or exhibit two or more of these properties. Reference may be had, for example, to German Utility Model No. 82 08 046 which discloses a conventional canned motor centrifugal pump wherein the driving unit transmits torque to the driven unit by way of a magnetic clutch. The can hermetically seals the driving unit from the driven unit so that the pump can convey an aggressive fluid medium which is permitted to enter the interior of the can but is maintained out of contact with the parts surrounding the can and/or with the surrounding atmosphere. An important advantage of canned motor pumps, including the pump which is disclosed in the German Utility Model, is that such pumps can operate properly without stuffing boxes or other types of seals and require no maintenance or a minimum of maintenance for long periods of time. The likelihood of leakage (while the can is intact) is negligible.
However, conventional canned motor pumps also exhibit a number of serious drawbacks, especially a concerns their versatility. For example, a conventional canned motor pump cannot readily convey a molten metallic or plastic substance or another flowable medium whose viscosity is high or extremely high. If the conveyed medium is a molten substance (e.g., a metal or a plastic material), the medium tends to solidify in the interior of the can (when the motor is arrested) and strongly opposes renewed starting of the driven unit to an extent such that the starting is impossible or that an attempt to restart the motor will entail serious damage to, or total destruction of, the driven unit and/or other parts including the can. The situation is analogous when the conveyed medium does not exhibit a tendency to actually solidify but merely a pronounced tendency to increase its viscosity in the absence of heating, e.g., in response to prolonged dwell in the interior of the can. Many substances are readily flowable only at elevated temperatures so that a relatively short interval of stagnation in the interior of the can at room temperature or even above room temperature will cause similar problems as the solidification of a molten metallic or plastic substance. The situation is aggravated if the driving and driven units are coupled to each other by a magnetic clutch (as in the aforementioned German Utility Model) because, when the driven unit exhibits a pronounced resistance to rotation with the driving unit (e.g., due to solidification or due to a pronounced increase of viscosity of conveyed fluid medium in the interior of the can), the driving unit merely rotates (slips) but cannot transmit torque to the driven unit.
Applicants are not familiar with any proposals to heat the can, probably because persons skilled in this art are aware of the need to establish very narrow clearances between the can and the adjacent parts of the driving and driven units.
It is further known to design a canned motor pump in such a way that a portion of the stream of conveyed fluid medium is diverted and is caused to flow along a predetermined path in order to lubricate plain bearings for the driven unit and to withdraw heat which is generated by the magnetic clutch and/or to withdraw heat which is generated as a result of frictional engagement of one or more plain (friction) bearings with the driven unit. The diverted portion of the fluid medium is returned into the main stream of such medium where it is likely to unduly raise the temperature of conveyed flowable material. Therefore, canned motor pumps wherein a portion of the conveyed medium is diverted for the purposes of lubricating and/or removing heat cannot be put to use in a number of important fields where an acceptable canned motor pump could perform the fluid conveying operation more satisfactorily than other types of pumps.
Still another drawback of conventional canned motor pumps is that the can is a highly sensitive part which is not always capable of standing long periods of use. For example, the can is likely to be damaged (so that it develops a leak) in response to damage to the bearing or bearings for the driven unit in the interior of the can. Moreover, the can is likely to be rapidly damaged or even destroyed as a result of contact with highly aggressive or corrosive chemicals and/or other conveyed fluid media. Still further, the presence of contaminants in the form of solid particles in the conveyed fluid medium is also likely to cause damage to (abrade) the can. For example, particles of ferrite are often a cause of serious damage to a conventional can. Such particles are attracted and entrained by the orbiting magnets of the clutch to thereby abrade the can in response to movement of these particles relative to the can and/or vice versa.
Still another potential cause of damage to the can are bearings which are made of or contain a ceramic material. Thus, while ceramic bearings exhibit numerous important advantages, they are brittle and are likely to be fragmentized in response to impacts and/or abrupt temperature changes. The fragments of ceramic material circulate relative to and thereby damage the thin-walled can.
It was further ascertained that the can of a conventional canned motor pump is likely to be damaged by the driven unit which is disposed in the interior of the can. For example, if the driven unit is allowed to run dry, it expands or undergoes other forms of distortion and comes in actual contact with the can.